This invention concerns chromatographic separations and, in particular, industrial scale chromatographic separations of sugars using a cation-exchange resin as the separating medium.
Chromatographic separations of various substances can be accomplished using ion-exchange resins as the stationary phase. Such processes use anion- or cation-exchange resins to separate mixtures of organic compounds, mixtures of organic compounds and salts, mixtures of acids and salts, and salt mixtures.
Of particular commercial importance is the separation of fructose from glucose and oligosaccharides in the production of high fructose corn syrup (HFCS). In this process, liquid mixtures of glucose and fructose are passed through one or more columns containing a strong acid type ion-exchange resin, typically in the calcium form. The passage of fructose through the column is not as rapid relative to that of glucose, so there can be obtained separate product streams containing high proportions of fructose and glucose respectively. The high fructose-containing stream may then be used as a sweetener for foodstuffs, such as soft drinks. This process is illustrated by Welstein and Sauer in "Separation of Glucose and Fructose: Effects of Resin Characteristics on Separation", in Ion Exchange Technology, Naden and Streat, eds. Society of Chemical Industry, London, at pp. 466-471.
The ion-exchange resin conventionally employed in separation of sugars is typically a plurality of cross-linked copolymer particles which contain cation-exchange functional groups. Previous to this invention, the resin was produced in a suspension polymerization process using a low to moderate level of cross-linker and no inert diluent. See, for example, U.S. Pat. No. 3,044,905. As used hereinafter, the term "conventional gel resin" refers to resins prepared from copolymer beads made without use of an inert diluent during polymerization.
Although good chrmoatographic separations of sugars are achieved using conventional gel resins, improvements which result in a faster, more efficient and/or higher yield operation are desirable. Thus, attempts have been made to modify the ion-exchange resin employed to improve kinetics.
In an otherwise similar process, use of resins having increased kinetics provide distinct advantages in comparison to use of conventional gel resins. For example, at a given product purity and yield, increased kinetics permit the column to be operated with a higher feed rate. Alternatively, increased kinetics result in higher product yields and/or purities when compared to use of conventional gel resins under substantially similar column operating conditions. Another advantage would be a reduced amount of desorbing solvent needed to elute the product from the column which reduces expenses associated with separating the desorbing solvent from the product. Increased kinetics further allow for use of larger resin particles, which permits a faster feed rate and/or higher feed concentration, thereby resulting in equivalent or better product yield and/or purity without increasing the pressure drop across the column. A resin having increased kinetics also permits an increase in column feed concentration to obtain a faster production rate.
In addition to kinetics, another important resin parameter is its flow characteristics, i.e., the ease with which a liquid mixture flows through a stratum, i.e., a column, of the resin. It is desirable that the mobile phase move rapidly through the resin at low pressures. The equipment normally used in a commercial chromatographic separation cannot withstand high pressures, so the flow rate cannot be greatly increased merely by increasing the pressure on the mobile phase.
It has been found that most modifications which improve the kinetics of conventional gel resins simultaneously diminish their flow characteristics. The kinetics of the resin can be improved by decreasing resin particle size, or increasing resin water retention capacity by reducing the amount of cross-linking monomer within the copolymer bead matrix. Unfortunately, both of these modifications diminish the flow characteristics of the resin so that, at a given pressure, a lower rate of flow of the mobile phase is obtained. Thus, benefits associated with any increase in kinetics are offset, in whole or part, by an undesirable reduction in flow characteristics.
Furthermore, a significant reduction in the degree of cross-linking is undesirable since it renders a resin more susceptible to oxidative degradation. Over a period of time, resins typically oxidize and degrade by decross-linking of the copolymer bead matrix. Loss of cross-linking results in resin beads which are softer and more elastic in nature, which tends to increase the pressure drop across a column of the resin with a corresponding decrease in flow rate of the liquid mixture being processed. Thus, a reduction in the amount of cross-linking will decrease the useful life of the resin.
Accordingly, it would be desirable to provide a process for the chromatographic separation of sugars from a sugar-containing liquid mixture using an ion-exchange resin as the stationary phase, wherein a faster, more efficient, and higher purity separation is achieved.